Biology/Natural History: Adult sand dollars move mainly by waving their spines, while juveniles use their tube feet. The tube feet along the petalidium are larger and are used for respiration. Tube feet elsewhere on the body are smaller and are used for feeding and locomotion. Individuals live subtidally, just beyond the surf, and flat on the bottom or at an angle and partly buried. They frequently move around if they are lying flat. If a current is present they usually lie at an angle and partly buried. Most individuals in the colony will be lying at a similar angle. In very rough water most lie flat and partly buried. Adults feed on detritus and diatoms swept to the mouth by cilia. If lying at an angle, it also catches small prey and algae with its pedicellariae, tube feet, and spines and passes them to the mouth. Their mouth includes a small aristotle's lantern. Predators include the seastar Pisaster brevispinus and the starry flounder Platichthys stellatus. Sometimes settled on by a small barnacle Balanus pacificus. Spawn in late spring and early summer. Fertilization is external. Juveniles swallow heavy sand grains, especially those with iron, which may help serve as a "weight belt" for them. May live 10-13 years. May be aged by counting growth rings on the plates of the test. Relative ages of individuals can be determined by counting the pores in a petal of the petalidium. Vaughn and Strathmann discovered that when pluteus larvae of this species are exposed to mucus from a fish predator, they begin to divide asexually by budding and fission. This results in a smaller average size for the larvae and also in a longer time for development. Vaughn and Strathman hypothesized that the adaptive value of this response may be that the smaller individuals are less likely to be eaten by the fish--a refuge in size. Presumably the slower development and longer time they spend as a pelagic larva, which is usually regarded as a disadvantage, is counterbalanced in this case by the temporary avoidance of fish predation. Steven C. Beadle notes that dendrasterid sand dollars such as this species are first found in late Miocene sediments in central California. They spread north to Alaska during the Quaternary and supplanted an abundant older fauna of symmetrical sand dollars.Hardy and Merz report that juveniles of this species, when inverted, spin around so that the posterior end is facing down-current. That orientation gives them the best configuration for being flipped back upright by the current.